1.1 Field of the Invention
The present invention relates to a method for accurate and reliable detection and quantification of leaks in pressurized pipe systems containing a liquid such as water, petroleum fuels and products, and other hazardous and nonhazardous substances. High performance is achieved because the method accurately compensates for the product temperature changes that occur during a test. The flow rate due to a leak, after the product temperature changes have been compensated for, is computed as part of the test. An apparatus for implementing the method on a pipe system is also described.
1.2 Brief Discussion of Prior Art
In U.S. Pat. Nos. 5,078,006; 5,090,234; 5,163,314; 5,170,657; 5,189,904; and 5,375,455, Maresca et al. describe a volumetric method for detection of leaks in a pressurized pipe system containing a liquid, and four general types of apparatuses to implement this method. The method and improvements to the method, which are described in U.S. Pat. Nos. 5,078,006 and 5,375,455, respectively, require that the pipe segment to be tested (1) be completely isolated from the remainder of the system by valves or valve blinds and (2) be pressurized to at least two different levels sequentially. These methods compensate for the thermal expansion and contraction of the liquid product in a pressurized pipe system and measure the difference in the volumetric flow rate due to a leak, if one is present, between two pressures. The main purpose of the apparatuses is to adjust and regulate line pressure and to measure the change in volume of the liquid product in the pipe system at a constant pressure.
1.2.1 Ambient Product Temperatures Changes
These methods and the apparatuses used to implement them are generally intended to test pipe systems that are located underground, but they can also be used on systems that are above ground or that are only partially buried. They can be used on lines containing any type of liquid. When applied to pipe systems containing petroleum fuels they have important advantages over other methods, because they can deal with large, nonlinear thermally induced volume changes. Petroleum lines experience large thermally induced volume changes because the coefficient of thermal expansion for petroleum fuels is large and the product temperature changes can also be large.
FIG. 1(a) is a time series showing the typical thermal behavior of product brought into a line at a warmer temperature than the backfill and soil surrounding the line; FIG. 1(b) shows the time series of the rate of change of temperature. The thermally induced volume changes are proportional to temperature, and they scale according to the volume of liquid in the line and the coefficient of thermal of expansion of that liquid. Thus, the two time series in FIG. 1 also illustrate the thermally induced volume changes that occur in the line. In this disclosure, we refer to this type of product temperature and product volume change as an ambient thermal change to distinguish it from other types of product temperature and product volume changes.
The observed curvature in both the temperature (or volume) and rate of change of temperature (or rate of change of volume) in FIG. 1 clearly illustrates the nonlinear changes in product temperature that occur during a test. When high performance is desired, testing with conventional volumetric methods, which do not compensate for the product temperature changes, cannot be initiated until the rate of change of temperature is such that volume changes are negligible. This means that the line must be taken out of service for whatever length of time is necessary to reach this stage of negligible thermal changes. Small lines at retail service stations may require a waiting period of 2 to 12 h, lines at bulk fuel storage facilities 12 to 36 h, and airport hydrant lines 1 to 3 days. This approach has adverse operational and performance implications. First, transfer operations may need to cease for an unacceptably long period of time. Second, there is no way to guarantee that a presumably adequate waiting period is in fact sufficiently long for thermal changes to dissipate. Third, even if the waiting period is adequate there is no way to verify quantitatively that the rate of thermal change is negligible or to verify that product temperature has not changed in response to other heat sources and sinks (e.g., heating or cooling of a section of an underground pipe that is exposed to sun or clouds).
The method and improvements to the method described in U.S. Pat. Nos. 5,078,006 and 5,375,455 are designed to compensate for the thermally induced volume changes produced by the ambient nonlinear changes in product temperature and thus to eliminate the need for the long pre-test waiting periods required by conventional testing methods. FIGS. 2 through 4 illustrate some of the pressure and measurement-period configurations commonly used for the conduct of a leak detection test; all of the configurations shown are based on three measurement periods. The configurations shown in FIG. 2 are described in U.S. Pat. No. 5,078,006, and the configurations shown in FIGS. 3 and 4 are described by U.S. Pat. No. 5,375,455. Each measurement period is numbered from 1 to 3 in the order of the data collection.
In the preferred application of the method, a test is conducted at two different levels of pressure, and the changes in the volume of product that occur at each of these two levels are measured at each of three equally spaced measurement periods (also referred to as measurement segments in this disclosure). In the first patent, two changes of pressure are required during a test, with the pressure during the first and third measurement periods being approximately the same. As described in the second patent, a test can be conducted with only one pressure change, with the pressure being maintained approximately constant during either the first and second measurement periods or the second and third measurement periods. This simplification, wherein the pressure can be the same in two consecutive measurement periods, has both operational and performance benefits. This improved method can be used (1) to minimize the volumetric transients that occur each time the pressure in the line is changed, (2) when storage, handling or disposal of product is difficult, or (3) for simplification of the test protocol. If one of the pressures described in either patent is atmospheric (zero gauge pressure), then a direct estimate of the flow rate due to a leak can be made at the other pressure. For some applications, more complicated test configurations are used, involving three or more pressures and/or four or more measurement periods.
If a leak is present, the volume data collected during each of the measurement periods are used to compute the difference in the temperature-compensated volume rate (TCVR) between the average flow rate due to the leak present in the first and third measurement periods, and the flow rate due to the leak present in the second measurement period. TCVR is computed by differencing (a) and (b), where (a) is the average of the volume rates measured during the first and third periods and (b) is the volume rate measured during the second period; the TCVR is also referred to as the test result. The volume rate (i.e., rate of change of volume, which is the first derivative of the volume data) measured during any given period is computed by dividing the volume change during a measurement period by the duration of the measurement period.
Stated in a general way, the method computes the TCVR from the difference between an estimate of the rate of change of volume during the second period (but at a different pressure), and the actual, measured volume rate during that same period. In the absence of a leak, the average rate of change of volume estimated from the first and third periods is a good estimate of the measured rate of change of volume during the second period, provided that the change in product temperature is not highly nonlinear. If a leak is present, then this difference yields a good estimate of the flow rate due to the leak in accordance with the pressure difference. This differencing method works well because it accounts for the nonlinear product temperature changes that occur during a test. The method is extremely effective if the second derivative of the volume data is approximately constant. The method is designed to compensate for the ambient thermally induced product temperature changes perfectly when the second derivative of the volume data is a constant. Degradation in performance occurs if the product temperature field is highly nonlinear (i.e., if the second derivative of the product temperature changes is not constant). Such problems occur if the product temperature field is very strong or if the total duration of the test is very long. Thus, the best accuracy is achieved when the total duration of a test is as short as possible (to minimize the magnitude of the nonlinear changes), and the duration of each measurement period is sufficiently long to make an accurate estimate of the rate of change of volume during each measurement period.
This differencing method is effective in detecting a leak because the rate of change of volume due to a leak varies as a function of pressure, whereas that due to the ambient thermal effects does not. The purely leak-induced volume change, which is expressed as a volume rate, will be constant at each given pressure but will differ from one pressure to the other. For a given hole in the line, leak-induced volume changes will be greater at a higher pressure than a lower one. The ambient product temperature field, except for small thermal perturbations that occur whenever the pressure in the line is changed, is unaffected by the pressure in the line. The differencing approach works because it compensates directly for thermally induced volume changes and does not affect the volume changes due to a leak.
For accurate test results, several constraints were placed on the test configurations illustrated in FIGS. 2 through 4. First, the duration of each measurement period had to be the same. Second, the intervals between periods, which generally differ in length from the duration of the periods themselves, also had to be equal. Finally, for the test configurations shown in FIGS. 2 and 3, the interval between the initiation of each pressure change and the beginning of the next measurement period had to be the same.
An estimate of the error in temperature compensation (referred to as the test error) can be computed directly if additional measurement periods are included at the beginning or the end of the test. In order to compute the test error, volume data must be collected at the same pressure in three consecutive measurement periods. The same differencing computation used for conducting a leak detection test is used to compute the test error. When the pressure is the same in all three measurement periods, the measured flow rate due to the leak will be zero. Any residual volume change that is measured will simply be the error in temperature compensation due to the presence of higher-order nonlinearities in the product temperature field.
1.2.2 Pressure-Induced Product Temperatures Changes
Additional operational and performance benefits could be realized by implementing the method with only two measurement periods. In U.S. Pat. Nos. 5,078,006 and 5,375,455, it was noted that the time interval between periods may need to be longer than the minimum time required to change pressure; this is to allow any transients and instabilities produced by the pressure change to subside so that accurate volume measurements can be made. The most important instability, the "small" temperature change associated with any pressure change, produces a perturbation in the underlying ambient product temperature field that degrades the performance of these methods. FIG. 5 illustrates (in an exaggerated way) the pressure-induced thermal perturbation resulting from increasing and decreasing pressure changes. The underlying ambient product temperature as it would have been had there been no pressure change is shown by the dashed line. These pressure-induced product temperature changes, which may be several hundredths to several tenths of a degree Centigrade, occur because the pressure change compresses the liquid or causes it to expand. An increase/decrease in pressure produces a small increase/decrease in temperature and vice versa. The degradation in performance occurs because the method assumes that the ambient product temperature changes occur monotonically during the entire test, and this assumption is violated by the small increase or decrease in temperature associated with the pressure changes during a test. In general, this is not a problem for tests conducted on small lines or over small pressure differences; however, it can be important when tests are conducted on large lines and over large pressure differences--for example, the petroleum fuel lines found at some bulk storage facilities and in most airport hydrant fuel distribution systems.
These thermal perturbations in temperature may take tens of minutes to many hours to come into equilibrium with the underlying ambient product temperature field. The magnitude of the temperature perturbation at a given point in time is dependent on the magnitude of the pressure change, the time that elapses between the pressure change and the next measurement period, the volume of product in the pipe system, and the system characteristics that control the rate of change of temperature of the product in the pipe (e.g., pipe diameter and pipe wall material, type of product in the pipe, and the type, characteristics, and condition of the backfill and soil surrounding the pipe). In many instances, after tens of minutes, the rate of change of temperature caused by these anomalous phenomena is too small to measure with most common temperature measurement sensing systems. However, if the line contains a large amount of product, the thermally induced volume changes may still be great in comparison to the size of the leak to be detected.
Pressure-induced thermal perturbations produce a systematic error, or bias, in the temperature-compensated volume rate computed from the volume data. For a given liquid product, the magnitude of this systematic error depends on the volume of product in the line, the difference between the low and high pressures used to conduct a test, and the time that elapses between any pressure changes and the subsequent measurement periods. If the liquid product in the line changes, then the magnitude of the systematic error also depends on the magnitude of the coefficient of thermal expansion of the liquid and the bulk modulus of the liquid. Whether or not this system error can be tolerated during a test depends on the performance desired of the system (i.e., the smallest leak to be detected). The systematic error can be reduced by reducing the volume of product in the line being tested, by reducing the magnitude of the pressure difference used in testing the line, by reducing the number of pressure changes required to conduct a test, or by increasing the time between any pressure change and the subsequent measurement period. This error can also be compensated for by calibration. All of these approaches, however, are dependent on each other, and each one of these approaches also has a number of drawbacks that can impact the performance of the method or its application to the particular line to be tested.
Reducing the volume of product in the line is effective, because the flow rate due to a leak does not scale with product volume and the volume changes due to the perturbation do. Dividing the line into smaller segments in order to reduce the effective volume of product, however, may not be possible, practical, or desirable. The maximum-size line that can be tested will depend on the performance requirements, the pressure difference, and the time between the pressure changes and the measurement periods.
Reducing the magnitude of the pressure change is not as effective as reducing the volume of product, because the perturbation and the volume changes due to a leak both scale with pressure. Such an approach is not always possible if the line must be tested at a prescribed pressure or if the pressure difference is not sufficient to detect the leak rate of interest.
Reducing the number of pressure changes is an effective way minimize the adverse effect of the thermal perturbation on the accuracy of a test. This can be accomplished, for example, by initiating a test at the existing pressure of the line.
Increasing the interval between any pressure change and the subsequent measurement period can be an effective means of addressing the adverse effects of the perturbation, since these decrease with time. However, if the duration of the test becomes too long, the accuracy with which the methods described above compensate for the ambient thermally induced volume changes is degraded. This is because the rate of change of temperature does not decrease linearly over long periods of time. Thus, for optimal performance, a balance must be found between the length of the intervals (between the measurement periods and the pressure changes) and the total length of the test.
The magnitude of the thermal perturbation can also be reduced by calibration. In a short test, the magnitude of this effect may be 2 to 10 times larger than the magnitude of the leak to be detected. In order to compensate for an effect of this magnitude, the calibration must be conducted with great accuracy. The calibration data must be collected on the line to be tested at the time when that line is known to be leak-free, i.e., when it has already been tested by another method. A calibration approach was described by Mertens in U.S. Pat. No. 4,608,857 for a pressure test, where the effects of these perturbations are described as "creep."
The performance and operational implementation of the methods described in U.S. Pat. Nos. 5,078,006 and 5,375,455 could be improved if: (1) the interval between any pressure change and any subsequent measurement period were increased, thereby reducing the effect of the thermal perturbation associated with that pressure change, and (2) the nonlinear changes in ambient product temperature, which become significant when the said interval is increased, were compensated for in such a way that the accuracy of the overall thermal compensation scheme was not affected by the duration of the test. If the latter improvement can be accomplished, the calibration requirement can be eliminated or at least reduced to the point where an analytical model or generalized database would predict the effects of perturbation with sufficient accuracy to allow the detection of small leaks.
1.2.3 Apparatuses for Testing Bulk Transfer Lines and Airport/Marine Hydrant Distribution Fuel Lines
U.S. Pat. No. 5,415,033 describes a simplified apparatus for implementing the method and for improving the accuracy of the volumetric measurements made during a leak detection test, but without the need for high-precision sensors and complex electromechanical systems. This simplified apparatus, which uses a passive means of maintaining constant pressure, can be used for testing applications in which very strict pressure control is not required. It can, for example, accurately test underground transfer lines at bulk storage facilities that may contain thousands of gallons of petroleum product. (It can also be used to test the small lines found at retail service stations.) For larger lines, like those that comprise airport hydrant fuel distribution systems, better pressure control and automation are required. The apparatus described in U.S. Pat. No. 5,090,234, which uses an active means of maintaining constant pressure during the volume measurements, can be used for testing these larger lines. This apparatus uses a pressure-feedback measurement system and a two-way pump (or two single pumps) for pressure management. A simpler, less expensive, and more accurate pump-apparatus for implementing these methods (which has application to these and other types of pipe systems) would be beneficial.